Airlines charge extra for the ‘better’ coach seats. It would be nice if they gave you something extra when you’re stuck in one of the seats that’s worse than average — a free cocktail, free inflight internet, or a discount on your next trip. At a minimum airlines could make the unwritten rule that the coach passenger gets priority for two arm rests explicit.
This isn’t a written rule, and it isn’t enforceable. We need to be courteous to fellow passengers, and this is the principle that gets us there.